Osteoarthritis (OA) is the most common adult joint disease. The frequency and severity of OA is increasing in all aging populations, with an estimated prevalence in the US of more than 25 million affected adults (Lawrence et al., Arthritis Rheum 2008; 58:26-35). Disease progression is associated with cartilage degradation, joint space narrowing (JSN), and loss of function. While radiographic progression has highlighted JSN, the emergence of magnetic resonance imaging has underscored the involvement of multiple joint tissues in OA, particularly subchondral bone, menisci, and synovium (Abramson and Attur, Arthritis Res Ther 2009; 11:227). The etiology of osteoarthritis is multifactorial involving both mechanical and biochemical factors. Osteoarthritis commonly affects the hands, feet, spine, and large extraspinal, weight-bearing joints, such as the hips and knees. The joints predominantly involved by osteoarthritis are weight bearing and include the knees, hips, cervical and lumbosacral spine, and feet. Other commonly affected joins include the distal interphalangeal (DIP) and proximal interphalangeal (PIP) joints of the hands. Primary osteoarthritis generally refers to osteoarthritis of no known cause. Secondary osteoarthritis generally refers to osteoarthritis resulting from some external or internal injury or disease (obesity, repeated trauma or surgery to the joint structures, abnormal joints at birth (congenital abnormalities), gout, diabetes and other hormone disorders). Generalized osteoarthritis affects many joints. Localized osteoarthritis typically affects a single joint, though in some cases, such as with finger arthritis, several joints may be affected. Accordingly, localized osteoarthritis may be said to affect the joints of one site.
Although OA has traditionally been considered a noninflammatory joint disease, it is now well-appreciated that mediators of inflammation are produced by articular tissues in OA and have been implicated in disease pathogenesis (Attur et al., J Biol Chem 2000; 275:40307-15; Pelletier et al., Arthritis Rheum 2001; 44:1237-47).